It has long been desirable to provide a system whereby drivers are alerted to the presence of emergency vehicles, such as those used by police and fire personnel, so that safe operation of passenger vehicles can be maintained.
U.S. Pat. No. 4,238,778 discloses an emergency vehicle warning system which includes an RF transmitter in the emergency vehicle and a receiver in other vehicles. The transmitter includes a single oscillator for generating a carder frequency of predetermined frequency. The receiver includes an antenna and bandpass filter, and the intensity of signals passed by the filter are compared to a predetermined intensity level to validate the signal and generate a warning.
U.S. Pat. No. 5,235,329 discloses an emergency vehicle detection system wherein a passenger vehicle includes a receiver sensitive to the frequency of a wave-borne signal generated by the emergency vehicle. The emergency vehicle transmits a signal in a broad frequency band, and the receiver located in the passenger vehicle generates an alert for any signals received within that frequency band. However, each emergency vehicle is configured to have a unique dead band somewhere in selected frequency band, and each emergency vehicle has a received designed to generate an alert only for signals it receives in that dead band. In that way, an emergency vehicle will be alerted to other emergency vehicles but will not alert based on its own signal.
Radar detectors, such as the ESCORT.RTM. and PASSPORT.RTM. radar detector products manufactured and sold by Cincinnati Microwave, Inc., assignee herein, are generally known and used by vehicle drivers for detecting the presence of police radar signals and generating an alert thereto, as disclosed in the following commonly assigned patents: U.S. Pat. No. 4,313,216; U.S. Pat. No. 4,581,769; U.S. Pat. No. 4,954,828; U.S. Pat. No. 5,049,885; U.S. Pat. No. 5,079,553; and U.S. Pat. No. 5,305,007. However, legislation has banned the use of such devices in many states and in interstate trucking. It has been proposed that such devices be used for other purposes, such as to detect emergency vehicles. However, no one has successfully developed such an application. Therefore, it would be desirable to utilize a radar detector to detect emergency vehicles and generate an alert for such detection.